feat(instances): Add PCIe slots for a VM that has none free

An existing q35 VM whose PCIe root ports are all used takes no new
virtio disk or network device while it runs, and the only way to give
it more was editing its XML.

The Disks tab of a PCIe machine now counts its slots, "PCIe slots: 0
free of 7", in red when none is free, with what its next start will
have when that differs, and Add PCIe slots adds four root ports to its
persistent definition: libvirt numbers and places them, and a running
VM gets them at its next start. The no-slot advice of Add disk and Add
network device points at it.

vrtManager/instance.py

@@ -706,6 +706,26 @@ class wvmInstance(wvmConnect):
     def get_machine_type(self):
         return util.get_xml_path(self._XMLDesc(0), "/domain/os/type/@machine")
 
+    def get_pcie_slots(self, config=False):
+        """(PCIe root ports, free ones) of the running or the persistent
+        definition, None when it has none (not a PCIe machine): each PCIe
+        device takes a port, the one whose index is its address's bus."""
+        tree = etree.fromstring(self._XMLDesc(PERSISTENT_XML if config else 0))
+        ports = tree.xpath("./devices/controller[@type='pci'][@model='pcie-root-port']/@index")
+        if not ports:
+            return None
+        used = {int(bus, 16) for bus in tree.xpath("./devices/*/address[@type='pci']/@bus")}
+        return len(ports), sum(int(index) not in used for index in ports)
+
+    def add_pcie_slots(self, count):
+        """Add count PCIe root ports to the persistent definition, which libvirt
+        numbers and places; a running VM gets them at its next start."""
+        tree = etree.fromstring(self._XMLDesc(PERSISTENT_XML))
+        devices = tree.find("devices")
+        for _ in range(count):
+            etree.SubElement(devices, "controller", type="pci", model="pcie-root-port")
+        self._defineXML(etree.tostring(tree).decode())
+
     def get_dom_emulator(self):
         return util.get_xml_path(self._XMLDesc(0), "/domain/devices/emulator")